1. Field of the Invention
The present invention relates to an optical characteristic measuring apparatus for obtaining an optical characteristic of a measuring object, particularly a transfer function matrix (for example, Jones matrix) of a measuring object, in details, relates to an optical characteristic measuring apparatus capable of accurate measurement even when a frequency difference of a first and second incident lights is varied.
The present invention relates to an optical characteristic measuring apparatus having an interference section for multiplexing a first input light and a second input light, frequencies of which differ from each other and polarized states of which are perpendicular to each other, inputting a multiplexed light to a measuring object, and making output light outputted from the measuring object interfere with at least one of the first input light and the second input light. The optical characteristic measuring apparatus obtains an optical characteristic of the measuring object, particularly, a transfer function matrix (for example, Jones matrix) of the measuring object by interference light from the interference section. In details, the present invention relates to an optical characteristic measuring apparatus capable of accurate measurement even when a frequency sweep speed of a waveform variable light source is not constant.
The present invention relates to an optical characteristic measuring apparatus for measuring an optical characteristic of a measuring object, particularly a transfer function matrix (for example, Jones matrix) of the measuring object, by branching light from a light source section, making one branched light incident on the measuring object, and making output light (signal light) outputted from the measuring object interfere with other branched light (reference light).
In details, the present invention relates to an optical characteristic measuring apparatus capable of easily determining an increase or a decrease of a phase difference of light (signal light and reference light) to be multiplexed.
2. Description of the Related Art
An optical characteristic measuring apparatus obtains optical characteristics (for example, insertion loss, reflectance, transmittance, polarized light dependency, wavelength dispersion, polarization mode dispersion, and the like) of a measuring object (for example, optical element, optical apparatus, test apparatus/measuring apparatus of the optical element or the optical apparatus or the like), specifically obtains a transfer function matrix (for example, Jones matrix) of a measuring object by measurement, and obtains the optical characteristics of the measuring object all together, or only the necessary optical characteristic from the transfer function.
In order to obtain the transfer function matrix by measurement, signal light having a frequency fs is made to be incident on the measuring object, and signal light (transmitted light or reflected light) outputted from the measuring object is multiplexed with reference light (frequency fr) to interfere with each other. Further, an interference signal is received by a light receiving section and an amplitude and a phase of the interference signal are measured (so-to-speak heterodyne detection). Further, in order to obtain a transfer function in a predetermined measuring wavelength range, a light source is subjected to wavelength sweep (frequency sweep) (refer to, for example, JP-A-2002-243585, U.S. Pat. No. 6,376,830, and JP-A-2004-20567).
FIG. 14 is a diagram showing an input/output characteristic to and from a measuring object 1. In FIG. 14, input light, output light to and from the measuring object 1 are represented by a column vector of 2 columns and 1 row (so-to-speak Jones vector) representing amplitudes and phases of two polarized light perpendicular to each other, and a transfer function matrix (so-to-speak Jones matrix) of the measuring object 1 is shown by Equation (1) as follows.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                                      ⁢                  (                                                                      T                  11                                                                              T                  12                                                                                                      T                  21                                                                              T                  22                                                              )                                    (        1        )            
In order to obtain such Jones matrix, first, second input light having polarized light (linearly polarized light, elliptically polarized light, circularly polarized light) polarized states of which are perpendicular to each other are inputted to the measuring object 1. Further, the amplitudes and phases of Jones vector of input light and output light outputted from the measuring object 1 are measured to obtain.
In order to easily obtain Jones matrix by operation from a result of measuring input light, output light, generally, linearly polarized light (for example, s polarized light, p polarized light) polarization planes of which are perpendicular to each other are used for the first, the second input light. Further, polarized states of respective input light of s polarized light, p polarized light to the measuring object 1 are changed by an optical characteristic of the measuring object 1 and emitted. Further, in order to facilitate the operation, in the output light from the measuring object 1, linearly polarized light (for example, s polarized light, p polarized light) polarization planes of which are perpendicular to each other are interfered with reference light to be measured.
That is, there are present emitted s polarized light and emitted p polarized light with regard to incident s polarized light, and there are present emitted s polarized light and emitted p polarized light with regard to incident p polarized light. Further, the incident s polarized light is s polarized light inputted to the measuring object 1, and the emitted s polarized light is s polarized light outputted from the measuring object 1. Also incident p polarized light, emitted p polarized light are similarly p polarized light inputted and outputted to and from the measuring object 1.
Therefore, in Equation (1), mentioned above, notation T11 represents a relationship of emitted s polarized light relative to incident s polarized light, notation T21 represents a relationship of emitted polarized light relative to the incident s polarized light, notation T12 represents a relationship of emitted s polarized light relative to incident p polarized light, notation T22 represents a relationship of emitted p polarized light relative to incident p polarized light. That is, in notation Txy, x represents a polarized state of an emitting side (x=1 represents s polarized light, x=2 represents p polarized light), y represents a polarized state of the incident side (y=1 represents s polarized light, y=2 represents p polarized light).
For example, when input light (that is, signal light) to the measuring object 1 is s polarized light, output light from the measuring object 1 becomes light multiplexed with T11 and T21 and becomes light multiplexed with T12 and T22 when input light is p polarized light.
In this way, it is necessary to measures polarized light, p polarized light having different polarization planes as input light and therefore, in a measuring method, there are a case in which measurement is carried out by subjecting input light to wavelength sweep by s polarized light and thereafter subjecting input light to wavelength sweep by p polarized light again, and a case in which measurement is carried out by one time wavelength sweep by simultaneously inputting s polarized light and p polarized light to the measuring object 1. When measured by one time wavelength sweep, a measuring time period can be shortened and measurement can be carried out accurately without an error derived from reproducibility (for example, wavelength reproducibility) in a first time and a second time of wavelength sweep.
However, since s polarized light and p polarized light are simultaneously inputted to the measuring object 1, it is necessary to separate an interference signal of s polarized light and reference light and an interference signal of p polarized light and reference light. In the separation, there are a method of separating by a time region by making the interference signal of s polarized light and the interference signal of p polarized light respectively constitute different measuring optical path difference (refer to, for example, U.S. Pat. No. 6,376,830), and a method of subjecting the interference signal of s polarized light and interference signal of p polarized light to intensity modulation respectively by different frequencies to separate from a difference in modulated frequencies for intensity modulation (refer to, for example, JP-A-2004-20567).
However, it is very difficult to separate the interference signal based on s polarized light and the interference signal based on p polarized light by the time region, when separated by the difference in the modulated frequencies, there poses a problem that a measured wavelength range is limited by wavelength dependency of an intensity modulator per se and the intensity modulator is very expensive.
[First Related Art]
FIG. 15 is a diagram showing a configuration of an optical characteristic measuring apparatus of a related art (refer to, for example, JP-A-2002-243585). In FIG. 15, a wavelength variable light source 2 outputs laser light while carrying out wavelength sweep by a predetermined wavelength sweep speed.
A half mirror (hereinafter, abbreviated as HM) 3 branches laser light from the wavelength variable light source 2 in two. A polarization beam splitter (hereinafter, abbreviated as PBS) 4 braches laser light in two of light (p polarized light, s polarized light) polarization planes of which are perpendicular to each other. Here, p polarized light is transmitted by an optical path OP1, and s polarized light is transmitted by an optical path OP2.
PBS 5 synthesizes light branched by PBS 4 and transmitted by the different optical paths OP1, OP2 to output to the measuring object 1. Here, light inputted to the measuring object 1 is signal light. A delay fiber 6 is provided on the optical path OP2 between PBS 4, 5 and delays one branched light.
Therefore, since incident s polarized light passes through the delay fiber 6, when a frequency of incident p polarized light is designated by notation f1 (t), a frequency of incident s polarized light becomes f2 (t) (f2 (t)≠f1 (t)). Here, respective f1 (t), f2 (t) are designated by notations f1, f2 as follows.
HM 7 synthesizes output light from the measuring object 1 and other light branched by HM 3 and transmitted by an optical path OP3. Here, light transmitted by the optical path OP3 is reference light. PBS 8 branches light multiplexed by HM 7 in 2 of light polarization planes of which are perpendicular to each other.
A light receiving section 9 receives one light (for example, p polarized light) branched by PBS 8. A light receiving section 10 receives other light (for example, s polarized light) branched by PBS 8. A light receiving section 11 receives light multiplexed by PBS 5. Further, PBS 5 receives light from a plane different from that emitted to the measuring object 1.
Therefore, at the light receiving section 9, three kinds of light of reference light (frequency f1′), emitted light polarized light (frequencies f1, f2) are interfered with each other. Further, reference light is provided with a frequency f1′ different from that of signal light, which is produced by an optical length difference of an optical path branched by HM 3 to the optical path OP1, PBS 5, the measuring object 1, HM 7 and an optical path of the optical path OP3, and the optical length difference is sufficiently smaller than an optical path length difference of the optical path OP1 and the optical path OP2 including the delay fiber 6. Therefore, a relationship of the frequency difference is represented by |f1′−f2|>>|f1′−f1|.
Naturally, when the optical path difference between the optical path branched by HM 3 to the optical path OP1, PBS 5, the measuring object 1, HM 7 and the optical path of the optical path OP3=0, the frequency f1=f1′.
Operation of the apparatus will be explained.
The wavelength variable light source 2 carries out wavelength sweep (frequency sweep) by a predetermined sweep speed. Further, HM 3 branches laser light from the wavelength variable light source in two. Further, a polarized wave controller, not illustrated, between the wavelength variable light source 2 and HM 3 pertinently controls polarized light such that laser light is branched in two at PBS 4.
Further, PBS 4 branches laser light to be multiplexed by PBS 5 by way of the optical paths OP1, OP2. One of multiplexed light is outputted to the measuring object 1 and other thereof is received by the light receiving section 11.
HM 7 synthesizes output light (signal light) from the measuring object 1 and other light (reference light) from the optical path OP3. Further, PBS 8 branches multiplexed interference light to two of linearly polarized light polarization planes of which are perpendicular to each other. Further, one light branched by PBS 8 is received by the light receiving section 9, other light is received by the light receiving section 10.
Further, filtering is carried out by a filter, not illustrated, at a rear stage and Jones matrix of the measuring object 1 is obtained by calculating section, not illustrated. As objects to be filtered, for example, at the light receiving section 9, there are present emitted p polarized light of frequencies f1, f2 and an interference signal by reference light of the frequency f1′.
Therefore, in order to obtain respective elements of Jones matrix by an output signal from the light receiving section 9, it is necessary to extract an interference signal of emitted p polarized light of the frequency f1 and reference light of the frequency f1′ and extract an interference signal of emitted p polarized light of the frequency f2 and reference light of the frequency f1′. Therefore, by a low-pass filter for passing a vicinity of a direct current component and a band-pass filter for passing a vicinity of a frequency difference |fl′−f2|, predetermined interference signals are provided and outputted to calculating section, not illustrated, at the rear stage. Further, the calculating section obtains Jones matrix. Further, by an output of the light receiving section 11, nonlinearity of wavelength sweep of the wavelength variable light source 2 is corrected.
[Second Related Art]
FIG. 16 is a diagram showing a configuration of an optical characteristic measuring apparatus of a second related art (refer to, for example, JP-A-2002-243585). In FIG. 16, a wavelength variable light source 2 outputs laser light while carrying out wavelength sweep by a predetermined wavelength sweep speed. A half mirror (hereinafter, abbreviated as HM) 3 branches laser light from the wavelength variable light source 2 in two.
A polarized light delay section 6c includes polarization beam splitters (hereinafter, abbreviated as PBS) 4a, 4b, and a delay fiber 6 for generating incident p polarized light, incident s polarized light from one laser light branched by HM 3.
PBS 4a branches laser light in two of light (p polarized light, s polarized light) polarization planes of which are perpendicular to each other. Here, p polarized light is transmitted by an optical path OP1 and s polarized light is transmitted by an optical path OP2. PBS 4b multiplexes light branched by PBS 4a and transmitted by the different optical paths OP1, OP2 to be outputted to a measuring object 1. Here, light inputted to the measuring object 1 is signal light. The delay fiber 6 is provided on the optical path OP2 between PBS 4a, 4b to delay s polarized light.
Therefore, since incident s polarized light passes through the delay fiber 6, when a frequency of incident p polarized light is designated by notation f1 (t), a frequency of incident s polarized light becomes f2 (t) (f2 (t)≠f1 (t)). Here, respective f1 (t), f2 (t) are designated by notations f1, f2 as follows.
HM 7 multiplexes output light from the measuring object 1 and other light branched by HM 3 and transmitted by an optical path OP3 to be interfered with each other. Here, light transmitted by the optical path OP3 is reference light. PBS 8 branches light multiplexed by HM7 into two of light polarization planes of which are perpendicular to each other.
A light receiving section 9 receives one light (for example, s polarized light) branched by PBS 8. A light receiving section 10 receives other light (for example, p polarized light) branched by PBS 8.
Therefore, explaining of the light receiving section 9, three kinds of light of the reference light (frequency f1′), emitted s polarized light (frequencies f1, f2) are interfered with each other. Further, the reference light is provided with a frequency f1′ different from that of the signal light owing to an optical path length difference between the signal light and the reference light.
That is, there is brought about an optical path length difference between an optical path branched by HM 3 to the optical path OP1, PBS 4b, the measuring object 1, HM 7 and the optical path OP3, the optical path length difference is sufficiently smaller than an optical path length difference between the optical path OP1 and the optical path OP2 including the delay fiber 6. Therefore, a relationship of a frequency difference is (|f1′−f2|>>|f1′−f1|).
Similarly, at the light receiving section 10, three kinds of light of the reference light (frequency f1′), emitted p polarized light (frequencies f1, f2) are interfered with each other.
Naturally, when the optical path difference between the optical path branched by HM 3 to the optical path OP1, PBS 4b, the measuring object 1, HM 7 and the optical path of the optical path OP3=0, the frequency f1=f1′.
Filter circuits 101, 102 are provided at a rear stage of the light receiving sections 9, 10 for subjecting signals from the light receiving section 9, 10 to low pass, band pass filtering. Calculating section 103 is inputted with signals filtered by the filter circuits 101, 102, (signal after low pass filtering and signal after band pass filtering).
Operation of the apparatus will be explained.
The wavelength variable light source 2 carries out wavelength sweep (frequency sweep) by a predetermined sweep speed. Further, HM 3 branches laser light from the wavelength variable light source in two. Further, a polarized wave controller, not illustrated, between the wavelength variable light source 2 and HM 3 pertinently controls polarized light such that laser light is branched in two at PBS 4a. 
Further, p polarized light, s polarized light transmitted by the optical paths OP1, OP2 to produce the frequency difference are multiplexed by PBS 4b to be outputted to the measuring object 1.
HM 7 multiplexes output light (signal light) from the measuring object 1 with other light (reference light) transmitted by the optical path OP3. Further, PBS 8 branches multiplexed interference light to two of linearly polarized light polarization planes of which are perpendicular to each other. Further, one light branched by PBS 8 is received by the light receiving section 9 and other light is received by the light receiving section 10.
Further, respective the filter circuits 101, 102 output signals from the light receiving sections 9, 10, or signals subjected to lowpass filtering, signals subjected to bandpass filtering to the calculating section 103. Further, the calculating section 103 obtains Jones matrix of the measuring object 1 from an amplitude and a phase of an interference signal after having been filtered.
Further, as signals to be filtered by the filter circuit 101, for example, at the light receiving section 9, there is present an interference signal by emitted s polarized light of frequencies f1, f2 and reference light (s polarized light) of the frequency f1′.
Therefore, in order to obtain respective elements of Jones matrix by an output signal from the light receiving section 9, it is necessary to extract an interference signal of emitted s polarized light (frequency f1) and the reference light (frequency f1′) and extract an interference signal of emitted s polarized light (frequency f2) and the reference light (frequency f1′).
Hence, the filter circuit 101 carries out the separation by a low-pass filter for passing a vicinity of a direct current component and a band-pass filter for passing a vicinity of the frequency difference (|f1′−f2|) and outputs the separated interference signal to the calculating section 103.
Similarly, as signals to be filtered by the filter circuit 102, for example, at the light receiving section 10, there is present an interference signal by emitted p polarized light of frequencies f1, f2 and the reference light of the frequency f1′.
Therefore, in order to obtain respective elements of Jones matrix by the output signal from the light receiving section 10, it is necessary to obtain an interference signal of emitted p polarized light (frequency f1) and the reference light (frequency f1′) and extract the interference signal of emitted p polarized light (frequency f2) and the reference light (frequency f1′).
Hence, the filter circuit 102 carries out the separation by the low-pass filter for passing a vicinity of a direct current component and the band-pass filter for passing a vicinity of the frequency difference (|f1′−f2|) and outputs the separated interference signal to the calculating section 103. Further, the calculating section 103 obtains Jones matrix from the interference signals from the filter circuits 101, 102.
[Third Related Art]
FIG. 17 is a diagram showing a configuration of an optical characteristic measuring apparatus of a third related art.
A wavelength variable light source 2 outputs laser light while carrying out wavelength sweep by a predetermined wavelength sweep speed. An optical fiber 366 transmits laser light from the wavelength variable light source 2. A lens 466 makes laser light emitted from the optical fiber 366 parallel light. A polarized wave controller 566 converts parallel light from the lens 466 to a desired polarized state (for example, linearly polarized light).
An interference section 666 includes a half mirror (hereinafter, abbreviated as HM) 666a, mirrors 666b, 666c, a polarization beam splitter (hereinafter, abbreviated as PBS) 666d, a polarization plane rotating section 666e, polarizers 666f, 666g, branches light from the polarized wave controller 566, inputs one branched light to the measuring object 1, and makes output light (signal light) outputted from the measuring object 1 interfere with other branched light (reference light).
HM 666a is a branching section for branching parallel light from the polarized wave controller 566 without depending on the polarized state, and outputs one branched light to the measuring object. The mirrors 666b, 666c are arranged on an optical path of other branched light branched by HM 666a, and successively reflect reference light.
PBS 666d is arranged on an optical path of output light from the measuring object 1, multiplexes reflected light (reference light) from the mirror 666c and signal light to be branched in two of light polarization planes of which are orthogonal to each other.
The polarization plane rotating section 666e is, for example, a ½ wave plate or the like, and provided between the mirror 666b and the mirror 666c. The polarizer 666f is provided on an optical path of one branched light from PBS 666d, and the polarizer 666g is provided on an optical path of other branched light from PBS 666d for making signal light and reference light interfere with each other.
A photodiode 766 receives interference light from the polarizer 666f of the interference section 666 and outputs a signal in accordance with optical power (also referred to as optical intensity) of received interference light. A photodiode 866 receives other interference light from the polarizer 666g of the interference section 666 and outputs a signal in accordance with the optical power of received interference light. A calculating section 966 is inputted with interference signals from the photodiodes 766, 866.
Operation of the apparatus will be explained.
In order to input respective p polarized light and s polarized light to the measuring object 1, wavelength sweep is carried out twice in a predetermined wavelength range. An explanation will be given from the first wavelength sweep.
The wavelength variable light source 2 outputs laser light while continuously carrying out wavelength sweep in a predetermined wavelength range. Further, the lens 466 makes laser light transmitted by the optical fiber 366 parallel light, and the polarized wave controller 566 converts a polarized state of laser light made to be parallel light into p polarized light to be outputted to the interference section 666.
Further, HM 666a branches light from the polarized wave controller 566, outputs one thereof to the measuring object 1 as signal light and outputs other thereof to the mirror 666b as reference light. Further, the polarization plane rotating section 666e inclines a polarization plane of reflected light from the mirror 666b by 45° to be outputted to the mirror 666c such that optical power is uniformly branched at PBS 666d at a rear stage.
Further, PBS 666d multiplexes output light (emitted s polarized light, emitted p polarized light in correspondence with incident p polarized light) from the measuring object 1 and reference light by way of the mirrors 666b, 666c to be branched in two of light (p polarized light, s polarized light) polarization planes of which are orthogonal to each other and outputs emitted p polarized light to the photodiode 766 and outputs emitted s polarized light to the photodiode 866. Further, polarization planes of light (signal light and reference light) multiplexed and branched by PBS 666d are orthogonal to each other and therefore, the light is received by the photodiodes 766, 866 by inclining polarization planes by the polarizers 666f, 666g. 
Thereby, the photodiode 766 is inputted with interference light of signal light operated by T22 of Jones matrix and reference light. Further, the photodiode 866 is inputted with interference light of signal light operated by T12 of Jones matrix and reference light.
Further, the photodiodes 766, 866 output electric signals in accordance with optical power of received interference light to the calculating section 966.
Successively, second wavelength sweep is carried out and a point of the second wavelength sweep which differs from the first wavelength sweep resides in that the polarized wave controller 566 converts laser light to s polarized light, that the photodiode 766 is inputted with interference light of signal light operated by T21 of Jones matrix and reference light, and that the photodiode 866 is inputted with interference light of signal light operated by T11 of Jones matrix and reference light, the other operation is similar to that of the first wavelength sweep and therefore, an explanation thereof will be omitted.
Further, the calculating section 966 calculates respective elements of Jones matrix from phases and amplitudes of interference signals based on respective p polarized light, polarized light to thereby calculate an optical characteristic of the measuring object 1 from calculated Jones matrix.
[With Respect to the First Related Art]
In the apparatus shown in FIG. 15, the frequency difference (|f1−f2|) of incident p polarized light, incident polarized light is determined by the optical path length difference of the optical paths OP1, OP2 and the wavelength sweep speed (frequency sweep speed), and also a frequency of a high frequency component of the interference signal is determined.
Therefore, the signal outputted from the light receiving section is separated into an interference signal of a high frequency component (several tens through several hundreds [MHz]) and an interference signal of a direct current through low frequency component (which is sufficiently lower than the high frequency component and is, for example, about DC through 200 [kHz]).
However, it is very difficult currently to subject a total wavelength range to wavelength sweep with linearity. Therefore, owing to nonlinearity of wavelength sweep, there poses a problem that a wavelength difference (frequency difference |f1−f2|) of p polarized light, s polarized light passing through the optical paths OP1, OP2 does not stay to be constant, the frequency of the high frequency component of the interference signal is varied and it is difficult to accurately obtain the optical characteristic.
Further, since the signal of the high frequency component is dealt with, in comparison with a case of dealing with the signal of the low frequency component, there poses a problem that circuit design of the band-pass filter for passing only the high frequency component, an electric circuit at a rear stage of filter and the like is difficult, and a circuit configuration becomes complicated.
[With Respect to the Second Related Art]
According to the apparatus shown in FIG. 16, the frequency difference (|f1−f2|) of incident p polarized light, incident polarized light is determined by the optical length difference of the optical path OP1, OP2 and a wavelength sweep speed (frequency sweep speed) and also the frequencies of the interference signals are determined.
Therefore, by filtering the signals outputted from the light receiving sections 9, 10 by the filter circuits 101, 102, an interference signal of a high frequency component (several tens through several hundreds [MHz]) and an interference signal of a direct current through a low frequency component (which is sufficiently lower than the high frequency component and is, for example, about DC through 200 [kHz]) can be separated.
However, it is currently very difficult for the wavelength variable light source 2 to subject a total measuring wavelength range to wavelength sweep with linearity. Therefore, owing to nonlinearity of wavelength sweep, a wavelength difference (frequency difference |f1−f2|) of p polarized light, s polarized light respectively passing through the optical paths OP1, OP2 does not stay to be constant, frequencies of the interference signals are varied to pose a problem that it is difficult to accurately obtain an optical characteristic unless characteristics of the low-pass filter, the band-pass filter are changed (for example, passing frequency bands are made variable).
[With Respect to the Third Related Art]
Jones matrix of the measuring object 1 is calculated from phases and amplitudes of interference signals of signal light and reference light.
However, generally, with regard to a light wave interference signal measured by the photodiodes 766, 866, signal intensity proportional to a trigonometric function of a phase difference of multiplexed light (signal light, reference light) is obtained to pose a problem that it is difficult to determine whether the phase difference is increased or decreased.
Therefore, for example, in U.S. Pat. No. 6,376,830, there is constructed a configuration in which the phase difference of signal light and reference light is only increased or decreased by bringing the optical path of transmitting signal light, the optical path of transmitting reference light, the optical path of the measuring object 1 under a predetermined condition. Therefore, there poses a problem that a configuration of the optical characteristic measuring apparatus is significantly restricted, that is, an optical path length of the measuring object 1 is limited.